<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title></title>
    <script type="application/javascript" href="http://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.0/jquery.js"></script>
    <style>
        body {
            /*颜色这个看个人喜好*/
            background: darkslategrey;
            /*字体这个看个人喜好*/
            font-family: sans-serif;
        }

        /*主要是规定中间表单尺寸，调整到自己觉得好看就行了*/
        .mainbody {
            height: 440px;
            width: 400px;
        }


        /*居中效果*/
        .middle {
            /*使左上角对应到父元素的中心*/
            top: 50%;
            left: 50%;
            position: absolute;
            /*向左向上偏移50%*/
            transform: translate(-50%, -50%);
        }

        .form-box {
            width: 100%;
            height: 100%;
            margin: auto;
            background: darkcyan;
            /* 我觉得这个圆角大小刚刚好 */
            border-radius: 40px;
        }

        .input-normal {
            width: 220px;
            height: 38px;
            margin: 30px auto;
            padding: 0;
            text-align: center;
            border-radius: 20px;
            outline: none;
            display: block;
            transition: 0.3s;
            border: 1px solid #e6e6e6;
        }

        .btn-submit {
            width: 100px;
            height: 36px;
            margin: auto;
            font-size: 18px;
            text-align: center;
            color: white;
            border-radius: 20px;
            display: block;
            background: darkslategrey;
            transition: 0.3s;
        }

        .front {
            transform: rotateY(0deg);
        }

        /* 将back旋转180度，背面朝向用户，我这边选y轴的 */
        .back {
            transform: rotateY(-180deg);
        }

        .front,
        .back {
            position: absolute;
            /* 然后设置为背面朝向用户时不可见 */
            backface-visibility: hidden;
            /* 我觉得用linear顺滑一点 */
            transition: 0.3s linear;
        }

        /* 将front旋转180度 */
        .middle-flip .front {
            transform: rotateY(180deg);
        }
        /* 将back旋转180度 */
        .middle-flip .back {
            transform: rotateY(0deg);
        }

        /* 我调整了一下，感觉还不错的样式，你可以改成其他的好看点的 */
        p {
            margin: 15px auto;
            padding: 0;
            font-size: 16px;
            color: white;
            display: block;
            text-align: center;
        }
        a {
            color: aqua;
            cursor: pointer;
        }
    </style>

</head>
<body>
<body>
<div class="mainbody middle">
    <form class="form-box front">
        <div>
            <h1>网盘登陆 </h1>
        </div>
        <div>
            <input class="input-normal" type="text" placeholder="UserAccount" />
            <input class="input-normal" type="password" placeholder="PassWord" />
            <button class="btn-submit" type="submit">
                <a href="http://www.baidu.com/"> 登陆</a>
            </button>
            <style>
                a{ text-decoration:none}
            </style>
        </div>
        <div>
            <p style="margin-top: 40px">If you don't have account.Please</p>
            <p>Click here to <a id="signup">注册</a></p>
            <script type="text/javascript">

                // 点击sigup触发翻转样式
                $("#signup").click(function() {
                    $(".middle").toggleClass("middle-flip");
                });


            </script>

        </div>
    </form>
    <!-- 对的你没看错，上下两个表单基本上没区别，一个front，一个back -->
    <form class="form-box back">
        <div>
            <h1>网盘注册</h1>
        </div>
        <div>
            <input class="input-normal" type="text" placeholder="UserAccount" />
            <input class="input-normal" type="password" placeholder="PassWord" />
            <button class="btn-submit" type="submit">
                注册
            </button>
        </div>
        <div>
            <p style="margin-top: 40px">Have a account ? You can</p>
            <p>Click here to <a id="login">登陆</a></p>
            <script type="text/javascript">

                // 点击login触发翻转样式
                $("#login").click(function() {
                    $(".middle").toggleClass("middle-flip");
                });

            </script>

        </div>
    </form>
</div>
</body>
</body>

</html>